Respiratory air flow sensor

ABSTRACT

The air flow sensor for a respiratory air flow measuring systems includes a tubular member for providing an air passageway for the flow of air between a patient and a ventilator. The passageway includes a fixed orifice-type obstruction comprised of a plurality of aerodynamically curved vanes which produce a pressure drop as the air flows through the passageway. The air flow rate of the patient&#39;s respirations is calculated by measuring the pressure differential between two longitudinally spaced apart pressure measuring air ports bored within one of the vanes in the air flow sensor. A third air port hole extracts respiratory gas samples for content analysis.

RELATED APPLICATIONS

The present application is a continuation-in-part of application Ser.No. 08/953,868, entitled Improved Pressure Differential Sensing Device,which was filed on Oct. 18, 1997 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a respiratory measurement system. Theprimary components of the system include a respiratory air flow sensor,a microprocessor based module, lumen tubing for connecting therespiratory air flow sensor to the module, a connector for connectingthe lumen tubing to the module, mechanism for optionally purging thesystem, and mechanism for optionally measuring content of a particularrespiratory gas. This application specifically concerns the respiratoryair flow sensor.

2. Background of the Related Art

A patient receiving anesthesia or in intensive care, for example, needsto have his or her inhalations and exhalations continuously monitored.Respiratory mechanics refers to the monitoring of the physicalparameters of a mechanically ventilated patient's airway. The parametersinclude airway flow and pressure. Various measuring devices are used tomeasure the air flow rate. For some patients the content of particularrespiratory gases flowing from or to the lungs must also be analyzed.

For measuring the air flow rate, it has been well known to use a tubulardevice which measures the pressure differential across a portion of thetube. An example of such a device is described in U.S. Pat. Nos.5,535,633 and 5,379,650 (referred to hereafter collectively as "the '633device"). The '633 device depends on the creation of a direct impedanceto the axial gas flow through the tube in order to obtain the pressuredifferential from which the air flow rate can be derived by theapplication of a certain nonlinear mathematical formula. The tube isformed from plastic and has an internal diameter or radius which ispartially blocked by a strut which obstructs the center of the airpassage. Such devices are classified as a fixed orifice air flow sensorbecause the internal geometry of the device is in fact fixed. Currentfixed orifice air flow sensors including the '633 device, however,present problems which arise from turbulence in the air flow through thesensor which causes a nonlinear response of pressure change versus airflow rate through the device. To account for the nonlinear response, the'633 system includes additional hardware which gain stages the pressurereadings. Current fixed orifice devices such as the '633 device also adda relatively high amount of resistance to the airway, which adds work tothe patient just to breathe.

Other known air flow measuring devices rely on a variable areaobstruction of the patient's respiratory air passageway. Such devicesare also tubular members which measure the pressure differential of theair flow through the tube. An example of a variable area obstruction airflow meter is described in U.S. Pat. No. 5,038,621 (referred tohereafter as "the '621 device"). The obstruction in the '621 air flowmeter is comprised of a flexible elastic membrane which extends into theflow stream. A portion of the membrane deflects as the air flows throughthe obstruction. Variable obstruction air flow sensors normally producea more linear pressure change versus flow rate measurement than do thecurrent fixed orifice-type sensors, but a variable obstruction sensoralso adds a relatively high amount of resistance to the air flow.Variable obstruction air flow sensors are also considerably moreexpensive to manufacture than are the fixed orifice type. The thinmembrane in particular is difficult to manufacture to consistently tightspecifications, so there is a significant amount of variability from onepart to the other. The '621 device is also made from multiplecomponents, which of course require assembly that naturally adds to thecost of the device.

To obtain an accurate pressure differential measurement, a great amountof resistance by the obstruction in the air flow sensor is desired. Thismust, however, be balanced with the fact that a high resistance addswork to the patient just to breathe, which is, of course, a reason tokeep the amount of resistance low. The internal geometry of the air flowsensor should also be of a type which provides the most accuratemeasurement possible over a range of air flow rates. Ideally a sensorexhibiting a linear or nearly linear pressure changes versus flow ratecurve through the full range of anticipated respiratory pressures andflow rates is desired.

A further feature of a respiratory measurement system is the connectorthat is used to attach the air flow sensor to the microprocessor basedanalyzer module. In known prior art systems the connector for connectingthe air flow sensor has been normally comprised of a first moldedreceptacle which is releasably connectable to a second moldedreceptacle, i.e., matching male and female receptacles. A typicalexample of such a device is the modular constructed connector disclosedin U.S. Pat. No. 5,197,895 (referred to hereafter as "the '895 device").The '895 device and other similarly designed male/female-type connectorsare normally quite expensive to treat as throw away or disposabledevices. An improved connector for connecting the air flow sensor to theanalyzer module, especially one which provides all of the necessaryfunctions required of a connector for a respiratory system of the typepresented here yet reduces or eliminates the number of components andthus reduces cost, is therefore desired.

Another feature of a respiratory measurement system is a means forpurging the system of condensation or other debris that may block theairways or block the lumen tubing which attaches the air flow sensor tothe analyzer module. In a patient ventilator circuit, natural cooling ofthe respiratory gases causes condensation of water vapor in the airtubes. If left undrained or unattended, the moisture will pool and mayclog the tubing connected to the air flow sensor. A ventilator works byperiodically compressing a volume of air which of course increases theair pressure in the ventilator circuit in order to force air into thepatient's lungs. The air is then returned to atmospheric pressures whichallows the patient to exhale. This continuously fluctuating pressure inthe breathing passage causes condensation in the ventilator circuit tomigrate into the lumen tubes that are used for measuring the pressuredifferential in the air flow sensor mentioned above. Of course, ablockage in the lumen tubes will cause errors in the air flowmeasurement. In most known prior art systems, the lumen tubes areperiodically purged with a short burst of air to clear any condensationor other obstruction that may be blocking the airway. One disadvantageof this method is that no measurements can be taken during the purge.Additionally, the purges normally occur at timed intervals, e.g., fiveminutes. In the event that a blockage occurs only one minute into theperiod, the air flow measurements will be incorrect for the remainder ofthe period. It is also difficult at times to determine whether ablockage has actually occurred because in some instances the signal mayappear like an actual breath. An improved method of purging the pressuremeasuring airway passages in a respiratory air flow sensor is thereforedesired.

In a respiratory measurement system it is also desirable to periodicallymeasure the content of particular respiratory gases. There are manytypes of gas analysis procedures, but one commonly used method isinfrared spectroscopy. In infrared spectroscopy, a sample of the gasextracted from the patient's respirations is passed through a gaschamber located between an IR emitter and an IR detector. Particulargases, such as carbon dioxide (CO₂) or nitrous oxide (N₂ O) are known toabsorb particular wavelengths of light. The presence and concentrationlevel of such a gas can therefore be determined by measuring the amountof light at the selected wavelength that has been absorbed by the gassample. Known IR analyzers are prone to consuming more energy than isreally necessary and inaccuracies due to unsatisfactory arrangement ofthe emitter and detector. An improved IR gas analyzer that betterutilizes and focuses the infrared light energy is therefore desired.Known IR analyzers are also prone to failure after a short period ofuse, and so a compactly designed field replaceable IR analyzer module isalso desired.

SUMMARY OF THE INVENTION

A respiratory measurement system is presented. The system is comprisedof a respiratory air flow sensor, a microprocessor based analyzermodule, a set of lumen tubes for connecting the air flow sensor to themodule, a connector for connecting the lumen tubing to the module, anelectronic subassembly for calculating the respiratory air flow ratebased on pressure differential measurements received from the air flowsensor, an infrared emitter and detector subassembly for analyzing thegaseous content of specified respiratory gases of a patient, and apneumatic subassembly for purging the system with a continuous lowpressure and low volume air flow.

The system includes three airway passages in communication with thepatient's ventilated respiratory airway that is being monitored. Two ofthe airway passages are used to measure two airway pressures values forcomputing certain respiratory parameters. These parameters includeinspired and expired respiratory air flow, pressure and volume. Thethird airway passage is used for extracting samples of the patient'sexhalations for analysis.

The air flow sensor is designed for placement into the ventilatorcircuit between the patient's endotracheal tube and the ventilator. Theairflow sensor is comprised of a tubular member providing a passage forthe flow of air between the patient's airway and the ventilator. The airflow sensor is a fixed orifice type sensor having a specified internalgeometry which creates a resistance to the patient's respirations.

The internal geometry of the air flow sensor is defined by thecylindrical walls of the tubular housing, and by a plurality ofelongated aerodynamically curved vanes which extend from the internalsurface to the cylindrical walls of the tubular member inwardly into theair passage. The internal geometry of the vanes creates a measurablepressure drop as air flows through the sensor. The unique internalgeometry for a flow sensor as presented herein produces a resistance tothe air flow which is far less turbulent than in current known fixedorifice sensors. The laminarization of the flow produces a nearly linearpressure versus flow rate response curve which greatly aids in thecalculation of the patient's respiratory air flow rates. In the presentinvention, the air flow rate is calibrated by measuring the pressuredifferential between two longitudinally spaced apart pressure measuringair ports within the air flow sensor. The sensor also includes a thirdair port for extracting a gas sample from the air flow for analysis.

The system further includes lumen tubing for connecting the air flowsensor to the analyzer module, and a novel connector for connecting thelumen tubing to the module. The lumen tubing is comprised of threetubes, two for transmitting the pressure signals and the third fortransmitting the gas sample from the air flow sensor to the analyzermodule. One of the tubes has an outer diameter which differs from theouter diameters of the other two tubes to ensure that the air flowsensor and analyzer module are connected together with the properpolarity with respect to the pressure changes being monitored.

The novel connector presented herein is comprised of a single moldedhousing specially designed to directly receive the triple lumen tubing.The tubing requires no special machining or preparation, the connectoris instead capable of accepting just the raw square cut ends of thetubing, and accepts the tubing only if it is attached to the connectorat the proper polarity. The matching male/female receptacles commonlyfound in the prior art are therefore unnecessary. The connector alsoincludes a means for determining a positive connection of the lumentubing to the connector, and a means for identifying of the particulartype of air flow sensor being used. In this regard the connector iscomprised two pairs of internal LED emitters and detectors strategicallylocated around the periphery of the connector housing. The emitters anddetectors are oriented such that when the clear plastic tubing iscorrectly placed into a socket in the connector housing, light energyfrom the emitters is refracted by the tubing and thereby directed toimpinge onto the detector. The first emitter and detector combination isused to determine the presence of the lumen tubing. When the tubing isnot present or not properly seated, the first detector remains dark andthe analyzer inactive. The second emitter and detector combination isused for identification of the particular air flow sensing device inuse.

The system further includes a pneumatic subassembly for purging thetubes and air sensor. The system places a very low, continuous flow ofgas into the lumen tubing. A low flow continuous purge reduces the depththat compressed gas from the ventilator enters into the lumen tubing,and also inhibits water droplets from forming near the air port openingsfor the lumen tubing in the air flow sensor. A continuous purgetherefore prevents obstructions from forming in the lumen tubing in thefirst instance.

The two lumen tubes used for measuring the pressure differential in theair flow sensor are both subjected to substantially the same continuouslow pressure air flow. Because the air flows being introduced by the twolines being purged are of the same pressure, the two added pressurevalues effectively offset each other so that the measured pressuredifference in the ventilator circuit being monitored remains unaffected.This approach also permits continuous monitoring without theinterruptions encountered in the periodic purge systems of the priorart. A continuous purge also employs less hardware, eliminates certainvalves necessarily required in a periodic purge system and thereforesignificantly reduces the cost of the system.

The system further includes a subassembly for analyzing certainrespiratory gases through infrared spectroscopy. Molecules of diatomicgases, such as carbon dioxide (CO₂) and nitrous oxide (N₂ O) absorb aspecific wavelength of light energy. If one passes CO₂ or N₂ O betweenan infrared emitter and an infrared detector, the amount of energydetected by the detector corresponds directly to the concentration ofthe gas of choice. The present invention includes a low power infraredCO₂ or N₂ O sensor that has a small physical size, has a very shortwarm-up time, efficiently dissipates heat generated by the light source,and therefore optimizes the battery life. The IR sensor further includesa gas sample chamber which effectively utilizes an elliptical reflectingsurface for the infrared light source. The light source and detector areboth oriented longitudinally along the axis of the ellipse and the lightsource and detector are each located at the two focal points of theellipse in order to optimize the light absorption reading.

The gas content analyzer additionally incorporates various compensatoryfeatures, allowing for automatic temperature and pressure compensation.For ease of use and serviceability, the present invention includes afield replaceable IR sensor subassembly which includes electronic memorychips for calibrating temperature and pressure constants used inrespiratory measurements.

Accordingly, the primary objects of the present invention are to providea respiratory measurement system which overcomes the problems of theprior art by providing an air flow sensor device having an internalgeometry which optimizes the process for measuring the air flow rate andgas content of a patients respirations; to provide a novel connector forconnecting lumen tubing from the air flow sensor directly to a gasanalyzer module; to provide within the module an electronic subassemblyfor analyzing and calculating the air flow rate of respiratory gases; toprovide in the analyzer module an infrared emitter and infrared detectorsubassembly for measuring the content of certain respiratory gases; toprovide a means for continuously purging a respiratory measurementsystem with a low volume air flow; and to provide a system whichincludes disposable or replaceable components which are highly costeffective in their design and manufacture.

Other objects and advantages of the invention will become apparent fromthe following description which sets forth, by way of illustration andexample, certain preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings, which constitute a part of the specification and includeexemplary embodiments of the present invention, include the following:

FIG. 1 is a plan view of the respiratory measurement system of thepresent invention;

FIGS. 2A, B, C, D, E, and F are top, front, bottom, rear, left side andright side views, respectively, of the air flow sensor of the presentinvention;

FIG. 3 is a perspective view of the air flow sensor;

FIG. 4 is a cross-section view of the air flow sensor;

FIGS. 5 and 6 are perspective view of the open ends of the air flowsensor;

FIG. 7 is a graph representing the air flow rate versus pressure withthe dashed line (middle line) representing the performance of the airflow sensor of the present invention as compared to a fixed orificesensor such as the '633 device and to a variable orifice sensor such asthe '621 device discussed above.

FIGS. 8 and 9 are perspective views of the tubing connector;

FIG. 10 is a partial sectional side view of the tubing connector;

FIGS. 11 and 13 are cross-sectional views of the connectors;

FIG. 12 is a cross-section view of the lumen tubing;

FIG. 14 is a cross-section view of the tubing connector showing the LEDemitters and detectors and the manner in which light from the emitter isrefracted by the lumen tubing and directed to the detector.

FIG. 15 shows the relative position of the infrared emitter and detectorof the IR analyzer;

FIG. 16 is a cross-section view of the infrared gas analyzer;

FIG. 17 is a perspective view of the infrared analyzer housing;

FIG. 18 is a top view of the flexible electronic circuit strip for theinfrared gas analyzer;

FIG. 19 is a perspective view of the replaceable infrared gas analyzersubassembly mounted to a circuit board;

FIG. 20 is an exploded view of the infrared gas analyzer subassembly;

FIG. 21 is a pneumatic diagram of the continuous purge system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings and in particular to FIG. 1, a ventilator (notshown) provides a source of air through a tube 20 to a patient 21.Patients connected to mechanical ventilators require an artificialairway placed in the nasal or oral endotracheal tube (referred to asintubation), or a tracheostomy tube placed directly into the tracheathrough a surgical opening in the neck.

A system for monitoring and measuring the patient's respirations iscomprised of an air flow sensor 22 for measuring the pressuredifferential of the air flowing through the ventilator circuit and forextracting gas samples for analysis, lumen tubing 23 for connecting theair flow sensor to a microprocessor based analyzer module 24, and aconnector 25 attached to the module housing for selectively connectingand disconnecting the lumen tubing to the analyzer module. Within theanalyzer module are, among other elements, an electronic subassembly forcalculating the air flow rate based on pressure differential signalsreceived from the air flow sensor 22, an infrared analyzer subassembly26 for analyzing the content of particular respiratory gases extractedfrom the patient's respirations, and a pneumatic subassembly 27 forcontinuously purging the lumen tubing 23.

1. Air Flow Sensor

Referring to FIGS. 1-6, the air flow sensor 22 consists of asubstantially hollow, generally cylindrical tubular member. The sensoris preferably fabricated from a common medical grade of polycarbonateplastic, which is quite inexpensive so that the air flow sensor 22 maybe disposed of after use by a single patient. The air flow sensor 22 hasa first open end 28 which leads to the patient 21 and a second open end29 which leads to the ventilator. The cylindrical configuration of thetubular member defines a longitudinal axis 30 through the center of theair flow sensor 22 in the direction of the respiratory air flow. Thehollow, cylindrical tubular member of course provides a passage for theflow of air between the ventilator and patient. In the preferredembodiment, the air flow sensor 22 has on one end a slightly smallerdiameter or cross section than the other end for connecting it tostandardized ventilator tubing 20. The cylindrical configuration of theair flow sensor 22 preferably defines a conventional 15 mm ID/OD airwayas required under ISO 5356-1. The nominal length of the sensor ispreferably about 2.5 inches.

The air flow sensor 22 is a fixed orifice type sensor having a specifiedinternal geometry which creates a resistance to the patient's inspiredand expired air flows. The resistance produces a pressure differentialbetween two longitudinally spaced apart air ports 31 and 33 in the airflow sensor 22. The measured pressure values are then used to calculatethe air flow rate of the patient's respirations.

The internal geometry of the air flow sensor 22 is defined by the innerdiameter of the cylindrical walls of the tubular member and also by aplurality of fixed internal vanes or fins 34, 35, 36 and 37circumferentially spaced apart around the inside of the sensor walls.Each vane or fin is comprised of an aerodynamically curved or arcuateshaped surface extending along the inner surface of the cylindricalwalls. The aerodynamic surfaces are elongated members orientedlongitudinally along the inner surface of the sensor walls in adirection parallel to the longitudinal axis of the tube. The aerodynamicsurfaces project inwardly from the walls toward the longitudinal axis 30of the tube, but are not connected to each other.

Each fin may be generally described as a pair of partial sphericalsurfaces 38 and 39 joined together to define a common line of symmetry40 which extends in a generally longitudinal direction through the airflow conduit. There are preferably four fins 34, 35, 36 and 37 withinthe tubular member with each fin spaced equidistantly approximately 90degrees apart from the next adjacent fin.

As such, the internal geometry is comprised of first 34, second 35,third 36 and fourth 37 aerodynamically curved surfaces extending fromthe inner surface of the cylindrical walls of the air flow sensor 22inwardly into the air passage. The first curved surface 34 extends fromthe inner wall of the tubular member a distance approximately equal tothe radial distance between the wall of the housing and the longitudinalaxis 30 of the tubular structure. The second 35, third 36 and fourth 37curved surfaces extend into the passageway a distance which is less thanthe radial distance between the wall and longitudinal axis 30 of the airflow sensor 22. Therefore, there is a space separating each of thecurved surfaces in the air flow sensor 22 from each other.

The first curved surface 34 in the air flow sensor 22 also has a firstair port 31, second air port 32, and third air port 33. The three smallair ports may alternatively be referred to as lumens in the sense thatthey are small bores in the side wall of the air flow sensor 22. Eachair port or lumen extends radially from the exterior surface of thecylindrical walls of the air flow sensor 22 radially inwardly into theair passage. The first 31 and third air ports 33 are longitudinallyspaced apart from each other and are used for measuring the pressure ateach port, respectively. As the patient inhales and exhales, air ofcourse flows through the air passage of the sensing device. The internalgeometry of the sensor creates a small pressure drop as the air flowsthrough the passage between the first and third air port 33. Thedifference in pressure between the first air port 31 and the third airport 33 is used to calculate the air flow rate.

The air pressure in the first and third air ports is transmitted throughthe lumen tubing 23 to the analyzer. Within the analyzer module 24 is anelectronic subassembly which converts the air pressure in the lumentubing 23 into an electronic signal representative of the pressuredifference between the first and third lumen tubes, and then through theuse of a mathematical algorithm calculates the air flow rate. Asmentioned, the novel internal geometry of the air flow sensor disclosedherein produces a highly desirable, nearly linear pressure versus airflow rate curve as reflected in the graph in FIG. 7.

Referring to FIG. 21, the pneumatic components for calculating the airflow rate in the analyzer module 24 include a pressure transducer 41, apressure differential transducer 42, and an electronic microprocessor43. The first air port 33 in the air flow sensor 22 is pneumaticallyconnected through the connector 25 (discussed further below) to thepressure transducer 41 and to the pressure differential transducer 42.The third air port 33 in the air flow sensor 22 is also pneumaticallyconnected through the connector 25 to the pressure differentialtransducer 42. The pressure transducer 41 obtains an absolute pressurereading of the patient's respiration and converts it to an electricalsignal which is transmitted to the microprocessor 43. The pressuredifferential transducer 42 determines the difference in pressure betweenthe first 31 and third air ports 33 in the air flow sensor 22 andconverts it to an electrical signal which is likewise transmitted to themicroprocessor 43. The microprocessor uses the signals obtained from thetwo transducers to calculate the patient's respiratory air flow rate.

The second air port 32 lies directly between the first and third airports, and it is used for extracting a sample of the patient'srespiratory gases for analysis, which is discussed further below.

The three air ports 31, 32 and 33 are preferably in the form of threesmall cylindrical recesses in the side wall of the air flow sensor 22.The recesses receive corresponding lumen tubing 44, 45 and 46 forconnecting the air flow sensor 22 to the analyzer module 24. Thediameter of the recess for the first air port 31 is slightly larger thanthe diameter of the second 32 and third air ports 33. For example, thediameter of the first cylindrical recess 31 is preferably about 0.1260inches, while the diameter of the second 32 and third cylindricalrecesses 33 are both preferably about 0.1050 inches.

The triple lumen tubing 23 is comprised of a first lumen tube 44, secondlumen tube 45, and third lumen tube 46, which are inserted into therecesses which form the first air port 31, second air port 32, and thirdair port 33, respectively. The diameters of the three lumen tubes ofcourse correspond to the diameters of the female recesses for the threeair ports, respectively. That is, the outer diameter of the first lumen44 is preferably about 0.1260 inches, while the outer diameters of thesecond 45 and third lumen tubes 46 are both preferably about 0.1050inches. The internal diameter of all three lumen tubes are identical,preferably about 0.0550 inches. Of course, different specific dimensionsmay be used so long as they are used consistently as outlined above.

When the patient exhales the air obviously flows through the sensor 22in one direction and when the patient inhales air flows in the oppositedirection. It is therefore imperative that the first 31, second 32 andthird air ports 33 be connected to the analyzer module 24 in the properpolarity so that the device records the inspired and expired air flowscorrectly. The different diameters of the recesses for the three airports thereby discriminate in receiving the different sizes of lumentubes in order to ensure the air flow sensor 22 is connected to theanalyzer module properly.

The air flow sensor 22 of the present invention is capable of measuringrespiratory air flow preferably in a flow range of about 1 to about 180lpm (i.e., about 16 to about 3000 ml/s) with an accuracy of about ±0.5%,or within about 0.5 lpm. The internal geometry described above adds anairway resistance of approximately only 2.0 cm H₂ O at 1 lps.

The air flow sensor 22 is a unitary plastic molded item which isinexpensive to manufacture. The internal geometry is defined by regularsurfaces that are easily fabricated, especially from molded plastic. Thesensor is of a one-piece construction which has no internal moving flapsor membranes. The pressure signals and gas samples are transmitted tothe analyzer module through a conventional medical grade of polyvinylchloride lumen tubing 23 which plugs directly into the novel connector25 described further below. Accordingly, the air flow sensor 22 andlumen tubing 23 together provide a highly reliable, very accurate,inexpensive, disposable respiratory air flow measuring device.

2. Tubing Connector

Referring to FIGS. 8-14, the connector 25 is especially adapted to actas an interface between the analyzer module 24 and the air flow sensor22. Specifically, the connector 22 provides a means for connecting thelumen tubes 44, 45 and 46 from the air flow sensor 22 to the analyzermodule 24. It further provides a means for discriminating the properpolarity of the lumen tubes and a means for identifying one of severaltypes of air flow sensors which can be used with the system.

The connector 25 is comprised of a connector housing 47 which isattached to the side of the analyzer module 24. The connector housing 47includes a female socket 48 for receiving the triple lumen tubing 23from the air flow sensor 22. The socket 48 is comprised of a firstportion 49 for receiving the first lumen tube 44, a second portion 50for receiving the second lumen tube 45, and a third portion 51 forreceiving the third lumen tube 46. The first 49, second 50 and thirdportions 51 of the socket 48 are each substantially cylindrical in shapeand each have a diameter corresponding to the diameters of therespective lumen tubes 44, 45 and 46.

Molded into the socket are three short stainless steel connector tubeswhich are also used for receiving the lumen tubes. Specifically, thereis a first connector tube 52 located in the center of the first portion49 of the socket 48 for receiving the first lumen tube 44, a secondconnector tube 53 located in the center of the second portion 50 of thesocket 48 for receiving the second lumen tube 45, and a third connectortube 54 located in the center of the third portion 51 of the socket 48for receiving the third lumen tube 46. The three metal connector tubesthereby provide three air passageways for connecting the three lumentubes to the internal components of the analyzer module. The three metalconnector tubes each have an outer diameter that is slightly larger thanthe internal diameter of the lumen tubes. For lumen tubes having aninternal diameter of about 0.055 inches as mentioned above, the metalconnector tubes preferably have an outer diameter of about 0.0575inches, thereby making the outer diameter of the metal connector tubesabout 0.0020 inches larger than the internal diameter of the lumentubes.

As mentioned, the lumen tubing 23 is preferably made of polyvinylchloride (PVC) which in this case is a somewhat flexible material. Thelumen tubes preferably have a hardness Durometer of about 75 to 80, andpreferably about 78. The metal connector tubes 52, 53 and 54 are ofcourse dense, extremely rigid metallic members. The lumen tubes andconnector tubes are sized such that, when one pushes the triple lumentubing into the socket 48, the relatively softer PVC material compressesso that the lumen tubes slip over the slightly larger rigid connectortubes fairly easily. Once the tubing is in the socket, the PVC material,which is no longer being compressed by the user, attempts to expand backto its normal size and shape, but at this point the wall of the lumentubing is compacted between the outer diameter of the metal connectortube and the inside wall of the socket. The compaction of the lumentubes within the socket provides enough frictional force to hold it inplace for normal usage. Consequently, the lumen tubing 23 fits fairlyeasily and snugly into the socket 48 and over the connect tubes, yet thelumen tubing may also be selectively disconnected from the module byyanking the tubing out of the socket. In this way, the air flow sensor22 and lumen tubes 23 are detachable and disposable, and the analyzermodule 24 is reusable by merely connecting a new air flow sensor to it.

The polarity that the air flow sensor 22 is attached to the connector 25is of course established by the order of and by the different sizes ofthe three lumen tubes 44, 45 and 46. The socket 48 in the connector 25for receiving the tubes is shaped to maintain the same polarity as theair flow sensor 22. In other words, the shape of the side walls of thesocket 48 corresponds to the outer diameter of the three lumen tubes sothat the lumen tubes 23 can be attached to the connector in only oneway. Specifically, the diameter of the first portion 49 of the socket 48is slightly larger than the diameters of the second 50 and thirdportions 51 of the socket. As a result, the first lumen tube 44 fitsonly into the first portion 49 of the socket 48, the second lumen tube45 fits only into the second portion 50 of the socket 48, and the thirdlumen tube 46 fits only into the third portion 51 of the socket 48.

The connector 25 further includes two pairs of internal LED emitters anddetectors 55, 56, 57 and 58 placed in lighting recesses positionedadjacent to the tubing socket 48. The emitter and detector pair notpointed directly at each other, but are instead angularly offset fromeach other so that when the tubing 23 is properly seated in the socket,light energy from the emitter is refracted by the tubing and therebydirected to impinge on the detector. When no tubing is present, lightenergy from the emitter is of course not refracted by the tubing and ismerely directed into the side wall of the socket 48. Therefore, unlessthe tubing 23 is present and properly seated in the socket 48, thedetector remains dark.

The first emitter and detector pair 55 and 56 are used in combinationfor the purpose of detecting the presence and proper seating of thelumen tubes 23 in the recess 48. The second emitter and detector pair 57and 58 are used in combination for identification of one of severaltypes of air flow sensors that can be used on the system. For instance,one type of air flow sensor may be especially designed for use on adultpatients, and a second air flow sensor may be specially designed andadapted for use on pediatric patients. The lumen tubing 23, or at leastthe ends thereof, may be color coded for identifying and distinguishingbetween different types of air flow sensors. Coloring dye on the end ofone of the lumen tubes will block the light energy emitted from one ofthe emitters from reaching the detector. Thus, activation of the firstemitter and detector pair determines the presence and proper seating ofthe lumen tubing in the socket, which is assumed to have an air flowsensor on the other end of the tubing, and thereby activates therespiratory monitoring system. Activation (or non-activation, as thecase may be) of the second emitter and detector pair identifies whetherthe air flow sensor is of the type used for adult patients or pediatricpatients, which in turn activates the appropriate program for theparticular type of patient being monitored.

The tubing connector 25 of the present invention thereby provides in asingle plastic molded part all of the features necessary to ensure apositive connection and identification of the air flow sensor 22 to themonitoring electronics in the analyzer module 24. The connector designdescribed above eliminates the need for a second male connector of thetype typically found in prior art monitoring systems.

3. Infrared Gas Content Analyzer

As mentioned, the second or middle air port 32 in the air flow sensor 23is used to extract a small sample of respiratory gases for analysisthrough the use of infrared spectroscopy. Such gas samples are of coursetransmitted from the air flow sensor 22 through the middle lumen tube 45to the analyzer module 24. The analyzer module 24 contains an infraredgas content analyzer subassembly 26 for analyzing the content ofspecified respiratory gases, normally either carbon dioxide (CO₂) ornitrous oxide (N₂ O). Referring to FIGS. 15-20, the infrared gas contentanalyzer subassembly 26 is comprised of a plastic molded analyzerhousing 59. The IR analyzer housing 59 is divided essentially into threecompartments, namely, a first compartment 60 for containing an infraredlight source 63, a second compartment 61 comprising a gas chamber whichthe gas samples are passed through, and a third compartment 62 forcontinuing an infrared detector 64. The gas chamber 61 is of coursebetween the infrared emitter chamber 60 and the detector chamber 62 andthe gas chamber 61 includes an inlet 65 for introducing the gas sampleinto the chamber and an outlet 66 for exhausting such gases 65 from thechamber.

The infrared emitter 63 is comprised of a broad band, high efficiencyinfrared light source 67 assembled onto an emitter housing 68. Theemitter housing 68 has a polished elliptical reflecting surface 69 forreflecting light from the light source 67 toward the detector 64 on theother side of the gas chamber. An ellipse of course has a centrallongitudinal axis, a first focal point and a second focal point. Thelight source 67 and the detector 64 are arranged in the housing 59 sothat the light source 67 is located at the first focal point of theellipse and the detector 64 is located at the second focal point of theellipse so that a maximum amount of light energy from the source isreflected by the elliptical reflecting surface 69 toward the detector64.

The infrared light source 67 is essentially comprised of an elongatedfilament having on each end an electrical connection. The filament isoriented so that it lies lengthwise along the central longitudinal axiswith a central portion of the filament positioned at the focal point ofthe elliptical reflecting surface. The longitudinal orientation of thefilament in this manner provides the greatest amount of light energy tobecome focused on the detector, thereby enhancing the reliability of thereading. Further, because the light energy is being used in the mostefficient manner possible, the amount of light energy and therefore theamount of heat generated by burning the filament is minimized, as is theamount of time necessary to heat up the infrared light source tooperational parameters.

The infrared detector 64 is actually a pair of detectors 70 and 71connected in series, the detectors being sensitive to heat from theinfrared source 67. The two detectors 70 and 71 are arranged incombination with a light filter 72 so that one detector 70 is subjectedto only light passing through the filter 72, and the other detector 71is altogether blocked from light from the infrared source 67. The lightfilter 72 may be comprised, in the case of CO₂ analysis, for example,from a thin sapphire window pane which passes through only a specificwavelength of infrared light which is naturally absorbed by CO₂. Thelight filter 72 consequently blocks all but the desired wavelength oflight. Therefore, the light striking the one detector 70 is limited tothe desired wavelength. The two detectors 70 and 71 are of coursesubjected to the same ambient temperature, pressure, humidity and soforth; the only difference between the two is the amount of infraredlight striking the one detector. Moreover, the amount of light strikingthe one detector is a direct function of the amount of light energywhich has been absorbed by the particular gas which is being analyzed.The two detectors 70 and 71 are therefore heated at different rates,which creates a small voltage between them which is in turn used tocalibrate the amount of CO₂, or N₂ O as the case may be, in the gassample.

Although the novel arrangement of the filament 67 disclosed aboveproduces a longer lasting light source, infrared light sources arenotoriously prone to burn out. The gas chamber 61 and infrared detector64 are also notoriously prone to blockages which if that occursobviously leads to incorrect results. The infrared gas analyzer 26disclosed herein is therefore uniquely constructed as a replaceablesubassembly.

Attached to the housing 59 is a flexible electronic circuit strip 73which provides all of the necessary electronic components for theinfrared analyzer, including electrical connections to the infraredlight source 63, electrical connections to the detector 64, barometricpressure and temperature compensating components 74 and 75 and an EEPROMmemory chip 76 for calibrating the sensor. The flexible electronic strip73 is a somewhat cross-shaped member which has a central portion 77attached to the bottom of the housing, a first flap 78 which folds overand is attached to a first side of the housing and at that point is alsoelastically connected to the light source 63 in the first compartment 60of the housing 59, a second flap 79 which folds over and is attached toa second side of the housing, a third flap 80 which folds over and isattached to a third side of the housing and at that point is alsoelectrically connected to the infrared light detector 64, and a fourthflap 81 which extends outwardly from the housing and terminates at anend which may be plugged into an electrical socket on a circuit boardwithin the analyzer housing.

Molded into the housing of the infrared analyzer are two metal connectortubes, namely, a first tube 65 which provides an air inlet for inputtingthe gas sample extracted from the patient's respirations by the airsensor into the gas chamber 61 in the housing 59, and a second tube 66which provides an air outlet for exhausting the sample from the gaschamber 61 into the atmosphere. The housing 59, further includes a meansfor fastening the housing to the circuit board, such as a threaded screw82 or other equivalent device which will adequately secure the housingto the circuit board yet also facilitate easy, in-the-field replacementof the housing. Thus, all of the essential elements for an infrared gasanalyzer, including the means to calibrate it in the field, are providedin one easily replaceable package.

4. Continuous Purge

The lumen tubes are purged by the introduction of a continuous lowvolume, low pressure air flow into the tubes. The continuous purge isproduced by a pneumatic subassembly 27 contained within the housing ofthe analyzer module 24. As mentioned, a pneumatic diagram of thepneumatic subassembly 27 for the continuous purge is illustrated in FIG.21. A continuous purge of the air flow sensor lumen tubes 31 and 33provides two unique advantages in comparison to the periodic purge foundin the prior art. First, a continuous purge reduces the depth ofcompressed gas coming into the sensor tubing. Secondly, the exit of thepurging gas from the lumen tubing into the air flow sensor 22 preventsor at least inhibits particulate H₂ 0 droplets from forming around theair orifice. This means that when the pressure surge from thecompressible volume effect happens, there will be less chance of largedroplets being drawn up into the tubing. Consequently, a continuouspurge of the air flow sensor lumen tubes prevents blockage fromoccurring in the tubes in the first instance, and further permits acontinuous monitoring of the patient.

The continuous purge approach of course requires balancing the purgingair flow. In other words, the values and pressure of the purging airflow must be equal into both sensor tubes 31 and 33. If the purge flowis not equal, errors will be introduced by the purge system into therespiratory measurement calculations.

Referring again to FIG. 21, the pneumatic components of the purge systeminclude an air pump 83, an air pressure reservoir 84, and two pneumaticresisters 85 and 86. As mentioned above, pressure readings from the airflow sensor 22 are pneumatically transmitted through the first lumentube 31 and third lumen tube 33 to the analyzer module 24. (The secondlumen tube 32, which is the middle tube and is used for transmitting thegas sample to the infrared gas analyzer, is not part of the purgesystem.) The air pump 83 and reservoir 84 provide a single source ofpressurized purging air for both the first 31 and third lumen tubes 33.

The air purge 83 and pressure reservoir 84 are pneumatically connectedthrough the first pneumatic resisters 85 to the first lumen tube 31, andconnected through the second pneumatic resister 86 to the third lumentube 33. The resisters 85 and 86 serve two essential functions. First,the resisters isolate the first and third lumen tubes from each other.The isolation factor permits the use of a single pump and reservoir forboth pressure lines, which simplifies the pump control. If thedifferential pressure lumens 31 and 33 were instead tied togetherthrough a single reservoir, without the resisters, the two pressurelines would automatically null each other out and no flow measurementcould be made. Second, the pneumatic resisters 85 and 86 provide a meansfor bleeding a small volume of low pressure air from the reservoir 84into the lumen tubes 31 and 33. The resisters 85 and 86 are preferablycomprised of a bundle of teflon coated conductors encapsulated inpolyvinyl chloride coating, e.g., 19 gauge multi-strand electrical wire.The insulating coating on the wire acts as a pneumatic conduit, and thebundle of wires within it severely restricts the flow of air through theconductor. Thus, a common electrical wire effectively acts as apneumatic resister which controls the amount of purge air introducedinto the system.

Because the amount of resistance provided by the pneumatic resisters 85and 86, is very high compared to the air pressure in the reservoir 84,and because the length of the first 85 and second pneumatic resister 86are substantially the same, the purge system introduces the purging airinto both the first 31 and third lumens 33 at substantially the same lowvolume and at substantially the same low pressure. This is importantbecause if the lumens were purged at different volumes or pressures,then an error would be introduced into the air flow rate calculations,which is of course undesirable.

Of course, specific details of the invention as disclosed herein are notto be interpreted as limiting the scope of the invention, but merelyprovides a basis for the claims and for teaching one skilled in the artto variously practice and construct the present invention in anyappropriate manner. Changes may be made in the details of theconstruction of the respiratory measurement system disclosed herein, andin the particular components of the system, without departing from thespirit of the invention, especially as defined in the following claims.

What is claimed is:
 1. An air flow sensor for measuring respiratory gas flow, said air flow sensor comprising:a tubular member, said tubular member having substantially cylindrical walls defining a passage with a first end opening and a second end opening for the flow of air between a patient's mouth and a source of air; a restriction within said passage, said restriction producing a resistance to said air flow, said restriction comprising a plurality of elongated aerodynamically curved surfaces extending from the cylindrical walls of the tubular member partially inwardly into the passage, each aerodynamically curved surface being spaced apart from the other aerodynamically curved surfaces; and, first and second longitudinally spaced air ports in one of the elongated aerodynamically curved surfaces for measuring a pressure differential of the air flow across said restriction.
 2. The air flow sensor of claim 1, wherein each elongated aerodynamically curved surface is comprised of an arcuate surface extending longitudinally along an inner surface of the cylindrical walls of said tubular member.
 3. The air flow sensor of claim 2, wherein each arcuate surface is comprised of a symmetrical pair of adjoining partial spherical-shaped surfaces.
 4. The air flow sensor of claim 1, wherein the plurality of elongated aerodynamically curved surfaces are spaced apart from each other circumferentially around the interior of the cylindrical walls of the tubular member.
 5. The air flow sensor of claim 4, wherein the elongated aerodynamically curved surfaces are spaced apart from each other equidistantly.
 6. The air flow sensor of claim 5, wherein a first elongated aerodynamically curved surface extends from walls of the tubular member inwardly into the center of the passage, and the other elongated aerodynamically curved surfaces extend from the walls of the tubular housing a distance less than to the center of the passage, thereby providing a space between the first elongated aerodynamically curved surface and the other elongated aerodynamically curved surfaces.
 7. The air flow sensor of claim 6, wherein the first and second air ports are each contained in the first aerodynamically curved surface.
 8. The air flow sensor of claim 5, comprising four (4) elongated aerodynamically curved surfaces in the passage.
 9. The air flow sensor of claim 1, wherein the first and second longitudinally spaced air ports each comprise a bore extending from an exterior surface of the tubular housing radially inwardly into the passage through one of the aerodynamically curved surfaces.
 10. A respiratory air flow sensor for measuring inspired and expired respiratory air flow rates, said respiratory air flow sensor comprising:a hollow tube structure, said hollow tube structure having substantially cylindrical walls having an internal diameter defining an air passage having a longitudinal axis; a plurality of fixed vanes spaced apart circumferentially around the internal diameter of the tube structure, each vane having an elongated aerodynamically curved surface oriented lengthwise in a direction parallel to said longitudinal axis, and each vane protruding from the cylindrical walls partially inwardly into said air passage so that each vane is radially spaced apart from the other vanes; and, in one of said vanes, first and second longitudinally spaced apart lumens for measuring the pressure differential of said air flow through said air passage.
 11. The respiratory air flow sensor of claim 10, said respiratory air flow sensor consisting of a one-piece plastic structure.
 12. The respiratory air flow sensor of claim 10, wherein the curved surface of each fixed vane is comprised of a pair of adjoining spherical-shaped surfaces.
 13. The respiratory air flow sensor of claim 10, wherein the vanes are circumferentially spaced apart from each other equidistantly around the cylindrical walls of the tube structure.
 14. The respiratory air flow sensor of claim 13, comprising four (4) vanes.
 15. The respiratory air flow sensor of claim 13, wherein the vane containing the first and second lumens extends inwardly a distance from said cylindrical walls to said longitudinal axis and the other vanes extend inwardly a distance less than to said longitudinal axis thereby providing a space between each vane.
 16. The respiratory air flow sensor of claim 10, wherein the first and second lumens have different diameters so as to discriminate in receiving one of a plurality of flexible lumen tubes.
 17. The respiratory air flow sensor of claim 16, further comprising a third lumen for extracting an air sample for analysis.
 18. An air flow measuring apparatus comprising:an air flow conduit; a first pressure sensing port in said flow conduit; a second pressure sensing port in said flow conduit, said second pressure sensing port being spaced apart from said first pressure sensing port; said first and second pressure sensing ports providing in combination a means for measuring a pressure differential of air flowing between said ports; and an obstruction in said flow conduit for resisting the flow of air therethrough, said obstruction comprising a plurality of fixed arcuate-shaped vanes within said flow conduit, each arcuate-shaped vane extending partially inwardly into the air flow conduit such that each arcuate-shaped vane is circumferentially and radially spaced apart from the other arcuate-shaped vanes in the flow conduit.
 19. The air flow measuring apparatus of claim 18, wherein the flow conduit is comprised of a one-piece substantially cylindrical tubular member.
 20. The air flow measuring apparatus of claim 19, wherein the plurality of arcuate-shaped vanes are comprised of four (4) vanes spaced circumferentially around the cylindrical tubular member approximately ninety degrees (90°) apart form each other.
 21. The air flow measuring apparatus of claim 18, wherein each arcuate-shaped vane is comprised of a symmetrically curved structure consisting of first and second partial spherical surfaces joined together to define a line of symmetry which extends in a generally longitudinally direction through the air flow conduit.
 22. The air flow measuring apparatus of claim 18, wherein the first and second pressure sensing ports are each comprised of a bore extending radially through one of the vanes from the exterior to the interior of the flow conduit.
 23. The air flow measuring apparatus of claim 22, wherein the first pressure sensing port has a diameter different from the diameter of the second pressure sensing port for discriminating in receiving one of a plurality of flexible lumen tubes.
 24. The air flow measuring apparatus of claim 23, further comprising a third radial bore for extracting a gas sample from the flow conduit.
 25. The air flow measuring apparatus of claim 24, wherein the third radial bore is located directly between the first and second radial bores.
 26. The air flow measuring apparatus of claim 25, wherein the third radial bore has a diameter which is the same as the diameter of the first radial bore but different from the diameter of the second radial bore.
 27. The air flow measuring apparatus of claim 18, wherein a first arcuate-shaped vane extends inwardly into the center of the air flow conduit, and the other arcuate-shaped vanes extend inwardly a distance less than to the center of the air flow conduit, thereby providing a space between the first arcuate-shaped vane and the other arcuate-shaped vanes. 